The Deep Space Network has a key role in the Deep Impact Mission - that of maintaining the link between the spacecraft and the earth. We use specially assigned radio frequencies for this link that are about a hundred times higher than we use to listen to FM radio stations.
The telecommunications link provides three main interactions that are vital to getting Deep Impact to its target.
- The tracking data measures the distance from the spacecraft to the earth; the navigators use these measurements to guide Deep Impact toward the comet Tempel 1.
- The command link provides a channel for control of the spacecraft and its computers and subsystems by the mission controllers at JPL.
- The telemetry link gives engineers a view into the operation of the spacecraft to ensure that all on-board tasks are being properly conducted. The telemetry link also brings the scientific measurements of the on-board telescopes and spectrometer to the science investigators.
The Deep Space Network conducts ground operations at its control center at JPL in the Space Flight Operations Facility and via the Deep Space Communication Complexes, which include the 34m and 70m antennas in the California desert, as well as those in Canberra, Australia and Madrid, Spain. In general, the DSN is the only link that deep space missions have with their engineering and science teams. The Deep Impact event is somewhat unique in that it is observable by earth-based and space-based telescopes so coordinating the encounter across a number of observing instruments adds to the complexity and to the richness of the results.
The Deep Space Network will provide continuous link support as Deep Impact approaches the comet. For the critical events such as important command sessions, spacecraft maneuvers, and observations of the comet the DSN provides redundant coverage from two stations. For the impact event itself, the 70m antennas in California and Australia will both have the spacecraft in view - this is fairly standard procedure for crucial deep space encounters. The impact event will also be viewed by an array of four 34m antennas in California to synthesize, in effect, a third 70m antenna. The real-time telemetry channel will be operating at 200 kilobits per second and every effort is being made to ensure that this unique data is captured regardless of weather or technical problems on the ground. After the close passage by the comet, a period during which the flyby spacecraft will play back the observations stored on board will complete our encounter coverage.